1. Field of the Invention
The present invention relates to an overrunning clutch which is installed in the power transmission path from a starter motor (to start, for example, an internal combustion engine) to a pinion so as to achieve the power transmission from the motor to the pinion.
2. Description of related Arts
An overrunning clutch for a starter comprises an outer member which is driven by the motor, and an inner member, which is driven by the outer member. The transmission of turning force from the outer member to the inner member is accomplishad by rollers arranged in cam cases between them such that the rollers squeeze themselves between the inner wall of the outer member and the outer wall of the inner member.
These rollers are required to work in such a manner that they squeeze themselves between the inner wall of the outer member and the outer wall of the inner member when turning force is to be transmitted from the outer member to the inner member and they slip between them when the internal combustion engine has started and the inner member has begun to run faster than the outer member (in the overrunning state) and hence it is necessary to prevent the transmission of turning force from the inner member to the outer member. For the rollers to move smoothly, the cam case is filled with grease. This grease, therefore, should have some coefficient of friction so that it permits the rollers to transmit turning force, and it should also have good lubricity to permit the rollers to slip without wear during overrunning.
The grease deteriorates in lubricating performance after the prolonged overrunning state in which the pinion is driven by the ring gear and the inner member runs faster than the rollers, resulting in a temperature rise.
Therefore, it is common practice to mix the grease with an extreme pressure additive which forms a chloride film upon reaction with metal, thereby improving the lubricating performance and preventing the lubricating performance from deteriorating due to the temperature rise that occurs at the time of overrunning.
Since the grease needs to permit the rollers to squeeze themselves between the two members and also to slip between the two members, as mentioned above, the extreme pressure additive becomes inactive at about 250.degree. C. and hence becomes incapable of maintaining the desired lubricating performance at higher temperatures.
In fact, a grease mix with an extreme pressure additive that exhibits good lubricating performance at high temperatures in excess of 250.degree. C. has such good lubricating performance that the rollers cannot squeeze themselves between the two members, with the result being that the transmission of the necessary turning force is impossible.) The consequence of continued overrunning with a deteriorated extreme pressure additive would be worn rollers. As the rollers wear due to deteriorated lubricating performance, the temperature rises to such an extent that the grease undergoes thermal oxidation.